1. Field of the Invention
The present invention relates generally to implantable medical devices for the treatment of osseous skeletal defects, and methods for their use.
2. Background Discussion of Related Art
In the past, skeletal defects have required amputation due to the associated “flail extremity” which prohibited weight bearing due to skeletal insufficiency and lack of effective muscle power. Early in the twentieth century, Lexer popularized the transplantation of large human joint (allografts) for such problems. However, these have been associated with high rates of infections, non-unions, accelerated arthritis, and mechanical complications. With the advent of hip prosthetics as developed by Austin Moore's proximal femoral prosthesis in the 1940's and John Charnley's low friction arthroplasty (total hip arthroplasty) in the 1960's and early 1970's, some of these problems were addressed in the hip, eliminating the problem of allograft joint degeneration.
The total hip arthroplasty was later combined with allografts, forming an allograft prosthetic composite (APC), taking advantage of the healing potential between the allograft and the residual host bone as well as the relatively painfree articulation of the total joint replacement. Concurrently, segmental prostheses or “tumor prostheses” were developed. The APC and segmental prosthesis were particularly needed in the era of “limb-preservation surgery”. This concept became possible with the development of chemotherapy agents that improved survival within the field of orthopedic oncology.
These allograft prosthetic composites (APC) were associated with high risks of infection and other complications. Massive osteoarticular allografts and APC's have a tremendous disadvantage due to some residual antigenicity and the slow incorporation of the allograft bone by host bone. The process termed “creeping substitution,” whereby the allograft bone is replaced by host bone in an appositional fashion, leads to overall weakening of the graft. Large allografts have been shown to be an “admixture of necrotic and viable bone.” This is in contrast to cancellous bone which based on its three dimensional porous architecture, facilitates bone ingrowth and increased mechanical strength after implantation.
Segmental prostheses are able to span the area of bone loss and are stabilized to the residual host bone. These prostheses, however, have several problems, including their large size, the high torques at the host-prosthesis interface, and risks of dislocation due to inadequate soft tissue attachments to the metal prosthesis. These issues are commonly found in the area of the knee and hip but also apply to the shoulder, elbow, ankle, and wrist. The search is ongoing for the ideal way to address a large segmental loss of bone adjacent to a large joint.
In some cases, due to bone loss resulting from infection or debris-mediated bone digestion, termed “osteolysis”, the residual bone allows a contained defect with thin but relatively preserved walls. In such cases, a technique known as impaction grafting has been developed and used since the late 1970's. The osseous defect is serially filled with layers of cancellous bone graft, which interlock due to the force of impaction. Into this newly formed cavity, a cemented prosthesis can be inserted. As the cancellous bone graft incorporates, it restores the patient's bone stock and provides an ongoing stable bed for the cemented implant.
The common complications with the technique relate to the loss of fixation due to fracture of the host bone or lack of containment and interlock of the cancellous bed. In some cases where the host bone has a segmental defect, it can be bridged with an allograft strut or some other containment device. Alternatively, metal mesh has been used to contain the allograft. However, use of such mesh is ineffective in the event of complete deficiency of the native cortical shell due to the lack of containment of the bone graft at the end of the construct, i.e., at the hip joint in the case of a proximal femoral deficiency.
A first representative prior art method and apparatus are shown in FIGS. 10A-10B herein. Here there is shown a femoral component prosthesis for hip arthroplasty that includes an implant shaft with a generally square cross-sectional shape. The shaft of the prosthesis is inserted into the residual proximal end of a resected femur and secured, either with cement or a press fit. However, as will be readily appreciated from even a cursory inspection of the schematic drawing, once implanted this prosthesis provides no means whatsoever for the reconstitution of bone at the site of the prosthesis. More importantly, there is no means for forming a column of reconstituted bone that surrounds the prosthesis. Removal of this prosthesis leaves only the resected femur as prepared for the initial prosthesis.
An exemplary prior art method and apparatus are shown in French Patent Document 2,315,902, to Blanquaert, et al, which is shown in FIGS. 11A-11B herein, and which teaches a metal rod for insertion into a femur for a hip prosthesis. The rod has a cruciform cross-section with four rails or ribs that define hollow zones into which bone growth material may be placed to facilitate bone regeneration. However, the metal mesh that contains the bone growth material is welded to the ribs, and when the rod is placed in a femur, the tips of the ribs engage bone endosteum. Accordingly, the reconstructed bone does not produce a contiguous and complete column of bone that surrounds the prosthesis. On the contrary, any reconstituted bone must emerge in a configuration of generally parallel fingers or spikes spaced apart by gaps or slots in the column, and removal of the prosthesis entails the removal of all of the reconstituted bone. Accordingly, removal of the joint prosthesis will not result in a free standing column of bone suitable for use in further reconstructive surgery. This is a significant shortcoming in this prior art method.